1. Field of the Invention
The present invention concerns a method and system to acquire an MR image of tissue containing iron oxide particles.
2. Description of the Prior Art
Iron oxide particles can be absorbed by cells and thus allow them to be marked. The iron can be detected in magnetic resonance tomography (MRT) with T2*-weighted gradient echo sequences that are very sensitive to magnetic field inhomogeneities. A disadvantage of the technique is that the iron oxide particles in the T2*-weighted image lead to signal losses. Therefore, areas with very low signal intensity in an MR image cannot be unambiguously ascribed to the iron oxide particles as long as other sources of signal losses (for example air pockets. (trapped air) or dephasing due to fast blood flow) cannot be precluded. A comparison acquisition before the administration of iron is thus always necessary. For this reason, measurement methods have been developed in which tissue containing iron oxide particles shows a positive image contrast. In the prior art, a number of methods are known that are all based on the fact that the iron particles generate microscopic magnetic dipoles. They generate both magnetic field gradients in their proximity and a shift of the resource frequency of the protons surrounding them. Spin echo sequences with non-selective 90° and 180° pulses are used in in Magnetic Resonance in Medicine, May 2005, 53(5):999-1005 by Charles H. Cunningham with the title “Positive contrast magnetic resonance imaging of cells labeled with magnetic nanoparticles”. However, the pulses are approximately 500 to 1500 Hz removed from the resonance frequency of free protons, such that only protons with strong shift of the resonance frequency in immediate proximity to the iron oxide particles contribute to the measurement signal.
The article by J. H. Seppenwoolde “Dephased MRI” in Magnetic Resonance in Medicine, 2006, 55(1):92-97, describes detection of tissue containing iron particles using gradient echo sequences with an unbalanced echo, i.e. with a dephased 0th gradient moment at echo time. The field gradients in proximity to the particles compensate the dephasing and, in the ideal case, result in a complete measurement signal, while the protons in a homogeneous magnetic field suffer from a severe signal loss due to the dephasing.
Furthermore, the article by M. Stuber “Positive contrast visualization of iron oxide-labeled stem cells using inversion-recovery with ON-resonant water suppression (IRON)” in Magnetic Resonance in Medicine, 2007; 58(5):1072-1077 states that the iron detection ensues by a frequency-selective preparation of the longitudinal magnetization before the imaging. Resonant proton spins that exhibit no frequency shift due to iron are specifically suppressed by special pre-pulses. However, this method—which is also similarly described in WO 2006/084125 A1—has the disadvantage that the T1 relaxation times of the water protons can locally fluctuate severely in the measurement subject, such that a complete suppression of the resonant protons is possible only in homogeneous tissue with uniform and known T1 relaxation time.